Always-on user equipment is expected to maintain packet data connectivity at all times and to always be reachable for push services. Further, the always-on user equipment must ensure that packet data protocol (PDP) contexts associated with its most important applications are maintained.
Data connectivity may however not be always automatically maintained or restored after a dual mode UE, that supports GSM and UMTS, performs a GSM to UMTS reselection or handover. More generally, data connectivity may not be maintained when the dual mode UE transitions between any network that does not support concurrent voice and data to a network that does support concurrent voice and data.
In the above-described transition, there may not be any instant trigger to cause signaling communication with the network, such as a change in location area code/routing area code (LAC/RAC). There may also be no outbound user data on the user equipment that would force the user equipment to send a routing area update message in case there was a RAC change. Such signaling messages are required to maintain or initiate restoration of PDP context.
In the case of the user equipment being in a voice call during a transition, a packet session that was suspended when the call was initiated in GSM or handed over to GSM may not resume until the voice call ends in UMTS. In this case, the user equipment will not be reachable from a packet data perspective even after it has entered a network that supports concurrent voice and data.
Further, if the user equipment performs a cell reselection from GSM to UMTS within the same routing area, according to 3GPP specifications (TS 24.008) there is no need to do an update if the UE is in packet mobility management (PMM) idle mode in UMTS until up-link user data or signaling information needs to be sent from the MS to the network, In addition, although 3GPP specifications (TS 24.008) indicate that after the successful completion of the handover from an GSM cell to an UMTS cell, a UE shall perform a normal RA update procedure in the UMTS cell in order to resume the GPRS services in the network before sending any other signaling messages or user data, 3GPP specifications do not mention when such update should take place. In such situations, if an always-on UE does not perform such update immediately after GSM to UMTS idle reselection or active voice call handover, UE may not be available from the perspective of push servers.
It is quite possible that the number of concurrent primary PDP contexts supported by a UMTS network is different from the number of concurrent primary PDP contexts supported by the GPRS network. For example, currently most UMTS networks support only one or in some cases a maximum of two concurrent primary PDP contexts, whereas GPRS networks support more than that. An always-on device needs to restore its most important PDP context such a context associated with its the push APN (access point name) context as soon as concurrent voice and packet data services are available.